The present invention is generally concerned with rotating machines such as motor vehicle alternators.
A conventional motor vehicle alternator is a polyphase generator generally including a stator within which turns a rotor provided with an excitation coil. The coil is energised via brushes in contact with two collector rings on a projecting part of the rotor shaft.
Using brushes has disadvantages, including the need for a relatively great axial length of the alternator, a set of brushes and collectors that increase the unit cost and malfunctions due to faulty contact between the brushes and the collector rings, in particular as a result of wear.
There exist in the prior art certain proposals aimed at providing rotating machines that can be used as motor vehicle alternators that have no brushes.
In one prior art alternator the claws of the two pole wheels of the rotor, which normally interpenetrate, are truncated to provide space in a transverse plane for a support for a fixed excitation coil disposed inside the pole wheels. However, this approach is detrimental to the efficiency of the machine, because the areas of the air gap are then very significantly reduced. What is more, for a given output/speed curve, a machine of the above type is significantly heavier than a conventional machine, which is particularly disadvantageous in the case of vehicle alternators.
In another prior art alternator two pole wheels with interleaved claws are mounted cantilever fashion at one axial end of the machine between an internal fixed excitation coil and external stator coils.
This prior art solution also has disadvantages, in particular the fact that the axial dimension and the weight of the machine are necessarily increased. Also, the enlarged air gaps in the rotor reduce the efficiency of the machine.
What is more, in both cases referred to above, the design of the machine makes it essential to take into account electromagnetic phenomena not only in the plane perpendicular to the rotation axis, i.e. in two dimensions, but also in the direction of the axis, i.e. in the third spatial dimension.
This necessity for three-dimensional design of the machine makes it extremely difficult and time-consuming to model and to optimise the various parameters.
A flux commutating machine with no brushes, known in particular from document EP-0 707 374, has the advantage of being easily modelled and optimised in two dimensions only.
This prior art machine nevertheless has the drawback of being restricted to single-phase operation, although three-phase machines can be desirable in a large number of applications, in particular in terms of electromagnetic efficiency and in terms of the simplicity and economy of the associated rectifier and smoothing means.